Late-time
interaction (725 ms) of a helium bubble in nitrogen gas with a
M=2.95 shock wave: The left side is an experimental image taken
from the shock tube; the image on the right is a computational
simulation with the code Raptor. It depicts the late-time interaction,
showing volume fraction and vorticity magnitudes.
(Larger image)

ecause
of their massive size, often-fleeting nature, and distance from Earth,
such celestial bodies and events as nebulae, galaxies, stars and supernovas
are difficult, at best, to understand. Scientists’ knowledge of
these astrophysical phenomena is limited primarily to observations they
glean from images like those sent back from the Hubble Space Telescope.

At UW-Madison, however, engineering physics researchers are conducting
unique, centimeter-scale experiments that may shed light on astronomical
events and processes that shape the universe.

The researchers’ tools include a 10-meter-tall shock tube, a couple
of gases, high-speed imaging technologies, and a 4-centimeter soap bubble.
“It turns out that the physics that happens at this scale is also
the same physics that happens at astrophysics scales,” says Senior
Scientist Mark
Anderson.

Upper
two-thirds of the shock tube

In the universe, a stellar
explosion, or supernova, triggers a shock wave that propagates through
the gas, dust and other media that comprise the matter that exists between
stars in a galaxy. Stars both form in and “feed” this interstellar
medium, a cycle that is crucial to a galaxy’s ability to support
active star formation.

To study the physics of gas-mixing that occurs during a stellar explosion,
the UW-Madison researchers re-create the hydrodynamics of a simple supernova
in the shock tube. Using a retractable stainless-steel “straw,”
they blow and float a soap bubble filled, for example, with helium,
inside the lower third of tube, which contains a gas such as nitrogen.
Then, they send a shock wave barreling down the tube. As the wave hits
the bubble, the two gases mix and a high-resolution camera captures
two-dimensional images of the event (illuminated by a laser pulse),
which lasts just thousandths of a second. A single experiment may yield
between one and three images; the researchers combine images from several
shock-tube runs to paint a more complete picture.

Filled with such geometric features as vortices and trailing plumes,
the shock-tube images mirror photos of actual astronomical events. “Using
the shock tube, with its very simple geometry of a spherical bubble
with a planar shock-wave interaction, we can help explain the physics
of some of those geometric features,” says Lecturer and Associate
Scientist Jason
Oakley.

Because they experiment with known, well-characterized quantities, the
researchers can extract meaningful data from the shock-wave images,
such as how vortex cores grow as a function of time or how the vortex
draws in surrounding gas. Recent diagnostic and technological advances
have enabled them to capture more detailed photos of the gases mixing,
and thus, to better understand the physics underlying their experiments.
“We’re able to experimentally identify features that have
never been seen before,” says Oakley.

Former
graduate student John Niederhaus and colleagues at Lawrence Livermore
National Laboratory, created this image, rendered from a 3-D multifluid
compressible Euler Scheme (Raptor). It shows the interaction of
a M=2.88 shock wave with an Argon bubble.
(Larger image)

Supercomputing advances also
have enabled scientists to develop and run complex, three-dimensional
computer simulations of the same features. In fact, University of Chicago
computational scientists are working with Oakley, Anderson and Professor
Riccardo
Bonazza to experimentally validate simulations of how different
gases react after they’ve been hit with a shock wave. As part
of that group, current graduate student Devesh Ranjan and former graduate
student John Niederhaus, now a scientist at Sandia National Laboratories,
conducted experiments and developed simulations that accurately predict
how two gases in the shock tube mix after the wave passes. The January
2008 Journal of Fluid Mechanics features the research on its
cover.

Although the physics of the researchers’ shock-tube experiments
translate well to astrophysics proportions, they also scale down just
as easily, says Anderson. Laboratories around the country are applying
the concept to learn more about how energy is released during inertial-confinement
fusion, a process in which lasers bombard and explode a pea-sized fuel
pellet. “Our work really compliments that research because it
simplifies the interaction and is at a much larger scale,” says
Oakley. “So, we can look at much finer features and understand
the hydrodynamic mixing.”